The patterning of radiation-sensitive polymeric films with high energy radiation such as photons, electrons or ion beams is the principal means of defining high resolution circuitry found in semiconductor devices. The radiation-sensitive films, often referred to as “photoresists” regardless of the radiation source, generally consist of multicomponent formulations that are usually spin-cast onto a desired substrate such as a silicon wafer. The radiation is most commonly ultraviolet light of the wavelengths of 436, 365, 257, 248, 193 or 157 nanometers (nm), or a beam of electrons or ions, or “soft” x-ray radiation, also referred to as “extreme ultraviolet” (EUV) or x-rays. The radiation is exposed patternwise and induces a chemical transformation to occur that renders the solubility of the exposed regions of the films different from that of the unexposed areas when the films are treated with an appropriate developer, usually a dilute, basic aqueous solution, such as aqueous tetramethylammonium hydroxide (TMAH).
Photoresists are generally comprised of a polymeric matrix, a radiation-sensitive component, a casting solvent, and other performance enhancing additives. The highest performing photoresists in terms of sensitivity to radiation and resolution capability are the group of photoresists termed “chemically amplified.” Chemically amplified photoresists allow for high resolution, high contrast, and high sensitivity that are not afforded in other photoresists. These photoresists are based on a catalytic mechanism that allows a relatively large number of chemical events such as, for example, deprotection reactions in the case of positive photoresists or crosslinking reactions in the case of negative tone photoresists, to be brought about by the application of a relatively low dose of radiation that induces formation of the catalyst, often a strong acid. The nature of the functional groups that comprise the polymeric matrix of these photoresists dictates the tone of the photoresist (positive or negative) as well as the ultimate performance attributes.
The nature of the polymeric matrix also dictates the suitability of a given photoresist for exposure with particular radiation sources. That is, the absorbance characteristics of a polymer must be carefully considered when designing a material for lithographic applications. This is important with optical lithography where polymers are chosen to provide a relatively transparent matrix for radiation-sensitive compounds such as photoacid generators (PAGs). Absorbance characteristics are also important because the wavelength of radiation used in optical lithography is directly proportional to the ultimate resolution attainable with a photoresist. The desire for higher resolution causes a continuing drive to shorter and shorter radiation wavelengths. For example, the phenolic polymers used for 248 nm imaging, namely derivatives of poly(4-hydroxystyrene) (PHS), are unsuitable for use with 193 nm radiation as the opacity of these PHS materials at 193 nm does not allow for sufficient radiation to create an appropriate image profile throughout the photoresist film thickness. That is, in order for photoresists to function properly, their films must be transparent enough at the exposing wavelength to enable sufficient light to penetrate to the bottom of the film to create usable developed relief images.
In addition to exhibiting the requisite transparency at a particular wavelength, it is important that a photoresist polymer be sufficiently polar so as to ensure solubility in industry standard developers. Polymers having lower solubility in these developers reduce the efficiency of resist development, a significant drawback in the manufacturing process.
There is, accordingly, a need in the art for a cost-effective and controllable method for incorporating functionality into polymers to impart desirable properties, including both polarity (and thus solubility in aqueous acid or base) and transparency at a particular wavelength. U.S. Pat. No. 3,444,148 to Adelman describes polymers prepared by direct polymerization of an alkene hexafluoroalcohol (i.e., an alkene containing a —C(CF3)2—OH group) with a variety of comonomers. The resulting copolymer compositions were found to have desirable characteristics relative to homopolymers that did not have an incorporated hexafluoroalcohol (HFA) group. This approach suffers from low incorporation of the desired HFA functionality (less than 2 mole percent) and is wasteful of valuable fluorinated monomer.
Incorporation of fluorinated alcohols into polymers for use in photoresist compositions has recently been described, but the availability of the requisite materials is limited. An attempt to incorporate an HFA moiety into polymerizable ethylene-containing monomers (vinyl ethers and some olefins) has been described in International Patent Publication Nos. WO 02/079287 A1, WO 01/86352 A2 and WO 03/040827 (DuPont). The methodology described in the aforementioned references involves the reaction of a heteroatom nucleophile with hexafluoroisobutene oxide. This restrictive chemistry limits the structural diversity of target molecules and is not suitable for the preparation of acrylate or methacrylate monomers.
Accordingly, there is an ongoing need for new compounds and methods that can be used to “tailor” the properties of a photoresist composition. Optimally, such compounds and methods would enable preparation of a broad range of polymer structures having desirable properties without need for costly starting materials or complex syntheses. The present invention is directed to the aforementioned need in the art, and, in part, provides compounds and methods that allow for the incorporation of fluoroalkylalcohol (i.e., fluorinated hydroxyalkyl or “fluoroalkanol”) groups in a cost-effective, controlled, reproducible manner.